1. Field of the Invention
The invention relates to the field of digital status signalling, and in particular concerns filtering the "trip" output of an electrical circuit protective device for operating a bell alarm or the like in a manner that distinguishes a real trip indication from short-lived pulses and signalling pulses on the trip signal. This is accomplished with a timed switching means that clears between pulses such that filtering is independent of the duty cycle of the pulses.
2. Prior Art
Devices for protecting electrical circuits and loads from fault conditions are often associated with controllable contactors that couple the load to the power line. In AC motor applications, the contactor arrangement is generally called a motor starter. The motor or other load may have a plurality of associated contact sets, operated electromagnetically to connect and disconnect the load and the line. In addition to on/off controls for activating and deactivating the load, a sensing circuit typically monitors the current passing between the line and the load, and often the line voltage as well. The sensing circuit produces a trip signal that triggers decoupling of the load from the line in the event of a detected fault condition.
The fault condition detected may be short term excess current due to a short circuit, long term excess current indicating overloading, a ground fault, phase imbalance or the like. Circuits for detecting such faults typically comprise some form of threshold responsive element generating a triggering signal that causes the contactor to disengage the power line.
The trip circuits of a number of associated contactors can be cascaded such that a trip signal generated by any of the associated contactors causes all of them to disengage their respective loads from the power line. It is generally undesirable to have the contactor trip circuits be so sensitive as to trip on the least pulse on the trip signal, which may be anomalous and not indicative of a real fault condition. Therefore, trip circuits are advantageously designed to trigger more slowly than they might, in order to reduce the incidence of nuisance trips.
In addition to cascading the trip circuits of contactors so they can be commonly tripped, contactors can be operated in a coordinated manner by coupling them to a controller. The controller is also responsive to the trip signal generated by the contactors. The Westinghouse Electric Corporation ADVANTAGE.TM. line of contactors, for example, includes a control module to which a plurality of contactors can be coupled for coordinated operation of loads such as three phase motors. The control module can include pushbutton controls for "run," "stop," "reverse" and the like, and effects coordinated operation of the contactors to make and break alternative connection configurations between the load and the line. A microprocessor in the control module monitors the status of the contactors via signalling paths.
The control module microprocessor can have a variety of functions such as timed sequence control of the contactors (e.g., to coast or stop before reversing, to start at slow speed and switch to fast after a delay, etc.). The trip signals of the contactors are cascaded, and the control module is responsive to a trip condition in its associated contactors. The control module can signal a trip condition to other control devices, and perhaps other control modules, for coordinated response to a trip condition.
The control module can be reset manually or by a signal applied to the microprocessor, which initializes the microprocessor by resetting its program address counter. The microprocessor has a number of initialization functions, but basically the microprocessor comes up when initialized in a mode wherein the contactors it controls are disengaged from the power line. It is thus possible to couple the trip output of the control module to the reset input of the control module as a way to respond to a trip. When a trip condition is detected causing the trip signal to change state, the microprocessor initializes and the contactors disconnect the loads from the power line.
The trip signal can be coupled to an indicator light for showing the status of the contactor/controller system. In one arrangement of the ADVANTAGE control module, the microprocessor pulses or blinks the trip indicator light and other particular indicator lights that normally are used to indicate "run," "stop" and similar modes, to represent certain states of operation. The control module tests its connections with the contactors as a part of the initialization routine executed after a reset, during which the associated signal lines are pulsed to blink the indicators. In another arrangement, the control module superimposes data pulses on a trip output to signal the status of its contactors. The outputs of the controller, and in particular the trip signal output, can thus routinely change state due to short pulses on the trip signal output due to initialization or other functions, which short pulses are not indicative of a fault condition in the load.
The threshold responsive sensing means associated with the contactor(s), for example a current threshold sensor, also may have certain timing considerations. For example, a direct short circuit as detected by sensing load current over a high current threshold should generate an immediate trip. A lower current threshold that exists for a predetermine, d time may indicate a thermal overload. In practical operation, the trip output of a threshold responsive means may pulse or change state a number of times as the detected current wavers around the threshold and finally remains above the threshold. Devices that are triggered by the threshold responsive means may respond fast, to any change of state on the trip output, or may be triggered only after the trip output has remained in the fault indicating state for more than a predetermined time.
One type of device that is advantageously triggered by such a threshold responsive means is a so-called bell alarm. The bell alarm may operate a bell, indicator light or the like, or simply generate a signal to a further control device. The bell alarm generally is activated when the trip output of the associated contactor or contactor controller changes into the fault indicating state. A relay or the like can be driven from the trip output for this purpose, or the trip output can operate a switching transistor or the like that activates a load device for announcing a trip condition or for signalling the trip condition. A typical bell alarm is triggered by the trip signal to set a latching relay, i.e., with one coil operable to latch the relay for coupling power to activate the alarm or signalling output device, and another to unlatch the relay when the device is reset.
However, problems are encountered due to the nature of the trip signal. Whereas the trip signal may be pulsed as the monitored current or other parameter approaches the triggering threshold, nuisance trips of the bell alarm occur. Also, if the system is one wherein the trip signal is pulsed during initialization and the like, a nuisance trip of the bell alarm may occur. To avoid such nuisance trips, it is possible to provide an additional output of the microprocessor that is not pulsed, and is triggered only after the trip signal changes to the fault indication level and stays there. This is an expensive solution to the nuisance trip problem.
In a typical motor control arrangement, the trip output is coupled to the reset input of a contactor controller via a circuit including an LED or similar local indicator light. A normally open manual reset pushbutton is connected in parallel with the local indicator light, and an RC timing circuit at the reset input provides a reset or power-up delay. In this circuit, pulsing the trip output by the controller apart from a trip condition, such as when starting or initializing, does not cause problems because the pulses are too short to trigger a reset. Moreover, blinking of the LED local indicator is a valuable diagnostic feature. In the event of a fault condition, however, the increasing duty cycle of the pulses, and eventually the steady state change in the level of the trip signal, raise the DC level at the reset input sufficiently to generate a reset.
A bell alarm triggered by this form of output signal transitions intermittently when starting or initializing, similar to the blinking of the indicator. The triggering and operation of tile bell alarm is erratic as the duty cycle of the pulses increases due to an impending trip. The operation of the bell alarm does not correspond with the occurrence of the trip at the controller (i.e., with the reset of the controller). These features are undesirable. It would be preferable to employ a circuit that could distinguish between short pulsing on the trip output and a steady state fault indication, and does not require an output of the microprocessor. An inexpensive and dependable circuit for distinguishing pulsing from steady state changes is needed. The circuit should use available signals (i.e., the trip signal), and should not unduly load the them, or affect the operation of the indicator light, manual reset pushbutton and controller RC reset input.